The CKS cell cycle regulatory proteins (CksHs1 and CksHs2 in human, suc1 in fission yeast and CKS1 in budding yeast) bind to the catalytic subunit of the cyclin-dependant kinases (Cdks as CDK2 or cdc2) and are essential for cell cycle progression in vivo. We have recently crystallized the CDK2-CksHs1 complex in two different crystal forms. A structure of this complex at higher resolution is critical to understand the Cks function. As CDK2 binds either human CKS or yeast homologs suc1 or CKS1, we also plan to crystallize CDK2 with one of the yeast CKS proteins to demonstrate the fine specificity of these proteins towards Cdks.